What's Considered Low Compression?

[undee70ss]

 

[RED-85-Z51]

My 330 started and ran okay, it felt okay, idled fine...visual inspection of the piston and cylinder showed like new parts. It didn't feel terribly impressive cutting but i was told the 330 was not an impressive saw.

So..i had the plug out and had my gauge 10 inches away, why not. 105psi...wayy below minimum spec. I tore it down and the rings had some 40thou end gap...rings were shot.

New rings going in...i expect a stark contrast

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[billyboy]

I don’t know squat about this but I have wondered why you wouldn’t want a warm or hot saw to test compression to better reflect running condition like the Homey info above. If your doing a cold saw would you want to set the choke for a few pulls to get some oil on the rings, then go to no choke with open throttle to get compression check? Isn’t the hot saw more realistic?

 

[RED-85-Z51]

I don’t know squat about this but I have wondered why you wouldn’t want a warm or hot saw to test compression to better reflect running condition like the Homey info above. If your doing a cold saw would you want to set the choke for a few pulls to get some oil on the rings, then go to no choke with open throttle to get compression check? Isn’t the hot saw more realistic?

In many cases you are testing cold because it won't start, so the tests remain consistent.

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[Oldtoolsnewproblems]

What has not been mentioned is that every time the rope is pulled even with the gage hooked up is that mix is being drawn into the cylinder, thereby raising the compression a bit, unless the compression is so low that it will not draw the mix in the first place. Think about it. Ken

That's a really good point, never thought about it. It also means if you're comparing across saws, the one with an empty fuel tank isn't getting a "fair" comparison to your daily driver saw that still has fuel.
Normally in a car (which I know much better than 2 strokes) I'd do a cold comp test, then squirt some wd-40 or whatever oil in a can is closest into the hole and repeat. That would give an idea if the low pressure was valves or the ring, but in a saw that's mostly useless. If you have very low compression with and without oil tho, is get suspect that your gauge isn't quite good enough for small 2 stroke engines. Some thick oil in the spark plugs hole will make even the worst ring make reasonable compression in my limited experience.

 

[Bob Hedgecutter]

For me- what I consider low compression is- if you know your fuel/air combination is correct and being delivered, you know the ignition is all strong and correct, you know the exhaust gasses are free to leave the combustion chamber and exit the exhaust- but the saw will still not start- well, that is considered low compression.